1. Field of the Invention
This invention relates to an image recording apparatus for effecting automatic optical power control (APC) over a semiconductor laser, a light emitting diode or the like.
2. Description of the Related Art
FIG. 1 is a diagram of an image forming operation of a conventional laser beam printer, and FIG. 2 is a cross-sectional view of FIG. 1.
An image signal (VDO signal) 101 is input into a laser unit 102, and the laser unit 102 outputs a laser beam 103 which is modulated in an on-off manner based on the VDO signal. A motor 104 rotates a rotating polygon mirror 105 at a constant speed to deflect the laser beam 103 into deflected laser beam 107 thereby to scan an area indicated by 107a.
An imaging lens 106 focuses the laser beam 107 on a photosensitive drum 108. Accordingly, the surface of the sensitive drum 108 is scanned with the laser beam 107 modulated with the image signal 101 in a horizontal direction (the main scanning direction). Referring now to FIG. 2, elements 102 to 106 are included in exposure unit 3. The sensitive drum 108 is rotated in the direction of the arrow and is uniformly charged by a charging roller 2 to which a high voltage is applied, and a latent image is formed by irradiation with the laser beam 107.
A beam detector 109 has a photoelectric conversion element 110 (e.g., a photodiode). The beam detector 109 outputs a horizontal synchronization signal (hereinafter referred to as "BD signal") 111 for determining an image writing timing.
The latent image formed on the sensitive drum 108 is visualized as a toner image by a development device 4. This toner image is transferred to a transfer sheet 112 by a transfer roller 5 and is fixed on the transfer sheet 112 by fixing rollers 6. Residual toner left on the sensitive drum 108 is removed by a cleaning device 7.
The signals for forming the image will be described below with reference to FIG. 3.
The BD signal 111 is a main scanning direction sync signal, as mentioned above. FIG. 3 shows the timing of outputs in the main scanning direction (horizontal direction) with respect to the transfer sheet 112. The image signal 101 is output a time t.sub.1 after the rise of the BD signal 111 to start forming the image at a distance D.sub.1 from the left end of the transfer sheet 112.
The image signal 101 is output from an image processing unit (not shown) such as an image processor that is different from a controller for controlling the image formation sequence. The controller effects masking by an image mask signal 113 so that no area outside the image area (outside the area defined by D.sub.2 in FIG. 3) is exposed even if the image processing unit turns on the image signal 101.
Since the beam detector 109 lies outside the image area, in order to generate the BD signal, it is necessary for the controller to forcibly light the laser at the time when the laser beam 107 moves across the beam detector 109. The signal used for this operation is an unblanking signal 114 (FIG. 3).
The mask signal 113 and the unblanking signals are generated by counting a system clock 124, as shown in FIG. 4.
The circuit shown in FIG. 4 will be described below.
The BD signal 111 from the beam detector 109 is formed as a pulse wave corresponding to one pulse of the system clock 124 by a waveform shaping circuit 123. The shaped BD signal is used to count a main scanning counter 122. The main scanning counter 122 counts up in synchronization with the system clock 124, and is cleared each time one pulse of the BD signal is supplied. That is, the position at which the laser beam 107 scans presently in the widthwise direction of sheet 112 can be found by detecting the value of the main scanning counter 122.
An unblanking start signal generating shift register 115 and an unblanking completion signal generating shift register 116 latch unblanking start data and unblanking completion data through data lines 127 and 128, respectively. Strobe pulses 125 and 126 are pulses used to latch the two registers 115 and 116. The contents latched by the registers 115 and 116 and the content of the main scanning counter are compared by comparators 117 and 118 to output to a flip flop 121 an unblanking start signal 129 through a gate 119 and an unblanking completion signal 130 through a gate 120.
An unblanking signal 114 is formed from these signals, as shown in FIG. 5.
The image mask signal 113 can also be formed by the same circuit structure as the unblanking signal 114 except that numerical values latched by the registers 115 and 116 are different.
In the above description relating to FIG. 1, it was simply stated that the laser unit 102 is turned on/off by the image signal 101, but it is, in fact, necessary to logically combine the image mask signal 113, the unblanking signal 114 and laser forcible lighting signal 131 to obtain the image signal 101 supplied to the laser unit 102, as shown in FIG. 6.
The image signal 101 can thereby be formed for the image area D.sub.2 alone. The laser forcible lighting signal 131 is a signal for enabling the controller arbitrarily to turn on the laser.
Next, automatic power control (APC) will be described. The relationship between the current supplied to a laser chip and the optical output varies with respect to individual chips and also varies according to the heat produced by the chip. For these reasons, laser emission cannot be effected by simple open-loop constant-current control. It is therefore necessary to control the laser unit by monitoring the optical output and maintaining a desired optical output level. This control is hereinafter referred to as APC.
APC will be described below in detail.
FIG. 7 is a circuit diagram of a laser control circuit.
This laser control circuit has a constant-current circuit 133, a switching circuit 135, an amplifier 138, and other components.
The constant-current circuit 133 constitutes a voltage/current converter through which a current I.sub.1 flows according to a light quantity control signal 134. The switching circuit 135 modulates this current in accordance with the laser lighting signal 132. A laser diode 136 emits light in accordance with the operation of the switching circuit 135. The quantity of light thereby emitted is detected by photodiode 137 which produces a current based on the quantity of light emitted by the laser diode. The current produced by photodiode 137 is converted into a voltage signal by a resistor 140.
The quantity of emitted light extracted as a voltage value is amplified by an amplifier 138 to be output as a light quantity signal 139. A comparator 144 compares the light quantity signal 139 and a voltage output from a reference voltage device 145 and outputs the result of comparison to an up/down counter 143. In conventional apparatuses, APC is conducted either during the unblanking period or during periods when the controller forcibly lights the laser diode. In this example, it is assumed that the apparatus has been configured to conduct APC during the forcible laser lighting period. Parenthetical references to the unblanking period are used in FIG. 7 to show the alternative configurations. The up/down counter 143 counts a clock signal CLK when the laser forcible lighting signal 131 (or the unblanking signal 114 in the alternative configuration) is output, and counts up or down according to the comparison result output from the comparator 144. The count value output from the up/down counter 143 is converted into an analog signal by a D/A converter 142. This analog signal is supplied as light quantity control signal 134 to the constant-current circuit 133 through a buffer 141. Thus, the detection output from the photodiode 137 is returned as a feedback current to the laser diode 136 to control the laser diode 136 during the forcible laser lighting period so that the quantity of light from the laser diode 136 is constantly maintained.
FIG. 8 is a flow chart of this APC operation using the laser forcible lighting signal 131.
For this control, the laser forcible lighting signal 131 shown in FIG. 6 is first activated and the light quantity signal 139 is thereafter monitored (step S1). If the quantity of light is smaller than a desired value, the level of the light quantity control signal 134 is increased by one step (step S2) or, if the quantity of light is higher than the level of the light quantity control signal 134 is reduced by one step (step S3). If the quantity of light coincides with the desired value, an unshown connection from comparator 144 signals the controller to terminate the laser forcible lighting signal 131, whereby the APC operation is terminated.
The area scanned with the laser beam during this operation relative to sheet 112 is as indicated by the arrows in FIG. 9.
This kind of APC is effected not only at an initial stage of the image formation operation (in a forward rotation period) but also in a non-recording operation period as between adjacent recording sheets if printing is effected on a plurality of recording sheets successively supplied.
In this process, however, the area between adjacent sheets is irradiated with laser beam and an unnecessary latent image is formed therein. The transfer roller is thereby contaminated and this contamination influences the recording image, that is, it reduces image quality and contaminates the back surface of the recording sheet. The conventional methods for preventing this problem require a complicated sequence of operation of charging the sensitive drum and reduce the throughput.
On the other hand, a method of effecting APC with respect to an area outside the image area as shown in FIG. 10 is possible. This method is used in a case where the desired light quantity level must be ensured every line or where the influence of the method relating to FIG. 9 upon the image formation is prominent. According to this method, the above-mentioned unblanking period and unblanking signal 114 are utilized.
However, the method utilizing the unblanking period entails a problem relating to the response of the light quantity signal 139 if it is applied to a high-resolution or high speed apparatus in which the unblanking period is short. For example, the quantity of light from the laser unit cannot be controlled unless the unblanking period is longer than a period t.sub.2 shown in FIG. 11, in which the light quantity signal 139 output converges to an output P.sub.0 corresponding to the output from the laser diode 136.
If the unblanking period is increased, the laser light strikes upon an edge or other portions of the polygon mirror 105, and the sensitive drum is irradiated with scattered light thereby caused, resulting in a considerable influence upon the image.